Hydrogen is utilized in a wide variety of industries ranging from aerospace to food production to oil and gas production and refining. Hydrogen is used in these industries as a propellant, an atmosphere, a carrier gas, a diluents gas, a fuel component for combustion reactions, a fuel for fuel cells, as well as a reducing agent in numerous chemical reactions and processes. In addition, hydrogen is being considered as an alternative fuel for power generation because it is renewable, abundant, efficient, and unlike other alternatives, produces zero emissions. While there is wide-spread consumption of hydrogen and great potential for even more, a disadvantage which inhibits further increases in hydrogen consumption is the absence of a hydrogen infrastructure to provide widespread generation, storage and distribution.
One way to overcome this difficulty is through the operation of hydrogen energy stations. At hydrogen energy stations, hydrogen generators such as reformers are used to convert hydrocarbons to a hydrogen rich gas stream. Hydrocarbon-based fuels, such as natural gas, LPG, gasoline, and diesel, require conversion processes to be used as fuel sources for most fuel cells. Current art uses multi-step processes combining an initial conversion process with several clean-up processes. The initial process is most often steam reforming (SR), autothermal reforming (ATR), catalytic partial oxidation (CPOX), or non-catalytic partial oxidation (POX), or combinations thereof. The clean-up processes are usually comprised of a combination of desulphurization, high temperature water-gas shift, low temperature water-gas shift, selective CO oxidation, selective CO methanation or combinations thereof. Alternative processes for recovering a purified hydrogen-rich reformate include the use of hydrogen selective membrane reactors and filters.
The gaseous hydrogen is then stored in stationary storage vessels at the hydrogen energy stations to provide inventory to fuel hydrogen vehicles. A cascade storage system is often used in the industry for dispensing gaseous hydrogen at hydrogen energy stations. The cascade storage system is divided into several storage banks of storage vessels. Several storage vessels with the same storage pressure are typically inter-connected to form one storage bank. In addition, several storage banks at different storage pressures are interconnected to form the cascade storage system. The cascade storage system has the ability to provide different consumers with different gas pressures in addition to increasing the utilization of the storage capacity.
Gaseous hydrogen is dispensed to hydrogen vehicles at a pressure of 5000 psi. In order to meet this requirement as well as the fast fueling requirement, gaseous hydrogen is typically stored in large quantities at pressures higher than the dispensing pressure. During the dispensing process, gaseous hydrogen is first withdrawn from the bank of storage vessels with the lowest pressure. Once the pressure of the vehicle tank of the hydrogen vehicle reaches that of the storage vessel, the gaseous hydrogen is next dispensed from the next bank of storage vessels at next higher pressure. This process continues until the vehicle tank of the hydrogen vehicle is full. The goal of the cascade storage system is to quickly and safely fill the hydrogen vehicle and to maximize the use of gaseous hydrogen.
Depending on the number of cascading storage levels, the hydrogen utilization rate is limited to about 35%-45% of the total onsite storage. The capital costs of retrofitting a storage system to achieve a utilization rate greater than 45% would not be economically viable due to the large number of valves in the system and the extra piping that would be required.
Another way to increase the utilization rate is to increase the number of cascading stages from the typical three stages to four, five, six, or any number of stages depending on the number of individual storage vessels that are available.
In addition, another way to increase the utilization rate is to increase the onsite storage pressure. Typically, to dispense at 5000 psi, the onsite storage pressure is approximately 6000 psi. In order to increase the utilization rate the onsite storage pressure can be increased to 6500 psi, 7000 psi, or higher. However, the cost of these high pressure vessels is high and availability is low.
The present invention addresses the desire to economically and efficiently increase the utilization rate of gaseous hydrogen stored in a cascade storage system.